As the rapid progress of portable and cordless home-use electronics in recent years, lithium-ion secondary batteries have been put in practical use as a power source of small electronic equipment such as laptop personal computers, cellular phones, and video cameras. Regarding the lithium-ion secondary battery, it has been reported by Mizushima et al. in 1980 that lithium cobaltate is useful as a positive electrode active material for a lithium-ion secondary battery. Since then, extensive research and development on a lithium-based composite oxide has been carried out, and many proposals have been made.
However, a problem of lithium secondary batteries using lithium cobaltate is degradation of cycle characteristics due to the elution of cobalt atoms or the like.
Further, the following Patent Literature 1 proposes a lithium secondary battery using, as a positive electrode active material, a lithium cobalt-based composite oxide in which a part of the surface of the particles of lithium cobaltate is coated with titanium oxide and/or lithium titanate in an amount of 2.0 to 4.0% by mole in terms of Ti.
Furthermore, the following Patent Literature 2 proposes a lithium secondary battery using, as a positive electrode active material, a lithium cobalt-based composite oxide in which the presence ratio of titanium on the surface of the particles of lithium cobaltate is 20% or more.
In the positive electrode active materials containing Ti atoms according to the above Patent Literatures 1 to 2, Ti atoms are present only on the surface of the particles of lithium cobaltate in high concentrations and hardly present in the inner part of the particles. Further, in the lithium secondary batteries using these positive electrode active materials, it is difficult to obtain a lithium secondary battery which is excellent in cycle characteristics and rate characteristics and low in direct current (DC) resistance and in which the swelling resulting from the generation of gas accompanying the reaction with a nonaqueous electrolyte solution is suppressed.